1. Field of the Invention
This invention relates to a method and apparatus for combining resin bonding and mechanical anchoring of a bolt in a rock formation and more particularly to an expansion shell assembly adapted for use with bonding material where mixing of the bonding material components and expansion of the shell take place upon continuous rotation of the bolt in one rotational direction.
2. Description of the Prior Art
It is well known in the art of mine roof support to tension bolts anchored in bore holes drilled in the mine roof to reinforce the unsupported rock formation above the roof. Conventionally a hole is drilled through the roof into the rock formation. The end of the bolt in the rock formation is anchored by either engagement of an expansion shell on the end of the bolt with the rock formation or adhesively bonding the bolt by a thermosetting resin to the rock formation surrounding the bore hole. The resin also penetrates into the surrounding rock formation to adhesively unite the rock strata and to firmly hold the bolt in position in the bore hole. The resin mixture fills the annulus between the bore hole wall and the rod along a substantial length of the rod. With a conventional expansion shell it is also known to chamfer the end of bolt so that the threads on the end of the bolt that receive the expansion shell terminate at a point spread from the entreme end of the bolt. This arrangement prevents damage to the threads to permit recovery and reuse of the bolt.
U.S. Pat. Nos. 3,324,662 and 3,394,527 disclose adhesively bonding a rod positioned in a hole drilled in a rock formation by a thermosetting polyester resin composition having thixotropic properties. It is well known that a bolt which is adhesively bonded in a bore hole can not be tensioned; on the other hand, a bolt mechanically anchored in a mine roof is capable of being tensioned but the contact of the roof bolt with the rock formation is confined to engagement of the expanded shell with the bore hole wall. Also, it is well known that deterioration of the rock formation surrounding the expanded shell reduces the contact area between the shell and the rock formation. Consequently the expanded shell slips and the tension on the bolt decreases, thereby reducing the roof support. Slippage of a tensioned mechanical roof bolt occurs most commonly in rock formations, such as shale, sandstone, mudstone, and the like.
In an attempt to resolve the disadvantages of anchoring by resin bonding or anchoring by expansion shells various types of mine roof support systems have been developed that combine mechanical anchoring and resin anchoring. The two systems have been combined by threading a bolt into a separate member such as a nut or coupling which is attached to a "rebar" anchored in the bore hole by a resin. A bolt with a plate held against the surface of the mine roof surrounding the bore hole is threaded into the separate member. Tightening the bolt places the bolt under tension.
U.S. Pat. No. 3,702,060 discloses an expansion shell assembly that includes a resin container which is fixed to the end of an expander positioned within an expansion shell. The container is ruptured after the shell begins to expand. Rotation of the bolt mixes the resin components, and the resin mixture surrounds the shell to embed the shell in the cured resin to bond the shell to the rock strata. When the resin is fully cured, a nut on the end of the bolt opposite a roof plate on the bolt is rotated to bring the roof plate to its fully seated position against the mine roof to fully tension the bolt.
Combining bolt tensioning and resin bonding of a mine roof bolt bolt in the bore hole is disclosed in U.S. Pat. Nos. 3,877,235 and 4,051,683. The devices disclosed in these patents utilize an internally threaded member such as a nut or coupling which is connected at one end to a "rebar" anchored within the bore hole by the mixed and cured resin. A bolt is then connected to the other end of the nut or coupling and includes a bearing or roof plate advanced into abutting relation with the mine roof. A stop means provided in the coupling limits axial advancement of the bolt into the coupling to prevent relative rotation of the coupling and the bolt as the assembly is rotated to break the resin cartridge and mix the resin components. When the resin cures the "rebar" above the coupling is adhesively bonded to the rock formation. Thereafter rotation of the bolt in the coupling fractures the stop means to permit the bolt to move upwardly in the bore hole so that sufficient torque is applied to the bolt to tension the bolt.
Similar devices which utilize a rod anchored within the drill hole by resin bonding and connected to a bolt by a coupling with a stop device to restrain relative rotation between the members of the assembly until the resin hardens so that the bolt can be tensioned are disclosed in U.S. Pat. Nos. 4,122,681 and 4,192,631. These devices rely upon the bonding of the elongated rod to the rock formation by the resin mixture. They do not utilize a mechanical anchor.
U.S. Pat. Nos. 4,160,614 and 4,162,133 disclose a mechanical anchor in combination with resin bonding of the bolt and the rock formation. Rotation of the bolt with the mechanical anchor attached to the end thereof in a first direction effects mixing of the resin components of a ruptured cartridge. An anti-rotation device prevents relative rotation between the camming plug and the bolt so that the plug is not threaded off the end of the bolt during mixing of the resin components. With this arrangement the resin components are thoroughly mixed before the shell is expanded. After a period of time sufficient for mixing the resin and before the resin hardens direction of rotation of the bolt is reversed to disengage the anti-rotation device. The camming plug is then free to advance downwardly on the bolt and expand the shell into gripping engagement with the wall of the bore hole.
The point anchor resin roof bolt support system utilizes the concept of anchoring a reinforcing rod in a mine roof by resin bonding and tensioning the bolt. The rod is anchored at its upper end in the bore hole by resin. A nut is positioned on the threaded end of the rod that emerges from the bolt hole and abuts a roof plate positioned in contact with the mine roof. The end of the rod at the nut is rotated to effect mixing of the resin. Rotation is terminated for a period of 30 to 60 seconds while the resin mixture cures. After the resin is set, then the bolt is rotated at a preselected torque to tension the bolt.
While it has been suggested by the prior art systems to anchor a roof bolt in a bore hole by combination resin bonding and bolt tensioning where mechanical anchors have been used it has not been possible to mix the resin and set the anchor by continuous rotation of the bolt in one direction. With the known systems the bolt must be rotated in a first direction to mix the resin while preventing expansion of the shell. When the mixed resin has begun to cure, then the direction of rotation of the bolt is reversed to expand the shell and set the anchor. Consequently careful attention must be given to rotating the bolt in the proper direction to mix the resin before the shell is set and not expand the shell before the resin is mixed. Furthermore when the bolt is rotated in the direction to effect mixing of the resin, necessary means must be provided to prevent threading the expander plug off the end of the bolt.
Therefore there is need in the system of combining resin bonding and mechanical anchoring of a bolt in a rock formation to provide a roof bolt anchor assembly that permits continuous rotation of a roof bolt in a single rotational direction to carry out both the operations of mixing the resin and expanding the shell.